1. Field of the Invention
The present invention relates to a vehicle heating apparatus having a heater duct disposed between an air blower unit and an air heater unit.
2. Description of the Related Art
Generally, heating apparatus for heating the passenger's compartment of vehicles such as automobiles comprise an air blower unit for discharging air at a given rate from air outlets in the passenger's compartment, an air heater unit with a built-in heater core for heating the air from the air blower unit before the air is discharged from the air outlets, and a heater duct disposed between the air blower unit and the air heater unit.
To convert such a heating apparatus into an air-conditioner, it has been customary to replace the heater duct with a cooling unit with a built-in evaporator. The heater duct, which is relatively expensive, is a dedicated part of the heating apparatus, and cannot be used in air-conditioners. Consequently, when a heating apparatus is converted into an air-conditioner, the removed heater duct is useless and highly uneconomical.
For this reason, there have been demands for heater ducts that can be used as cooling unit cases of air-conditioners, and various proposals have heretofore been made to meet such demands.
For example, Japanese laid-open utility model publication No. 64-37710 discloses a heater duct (hereinafter referred to as "prior art 1") that can be used as a cooler unit housing an evaporator therein, the heater duct having a flow-rectifying plate positioned therein fully across its cross section at an intermediate position and having a number of air passages for uniformizing an air flow delivered from an intake unit. According to Japanese laid-open patent publication No. 5-38928, a resistive body (hereinafter referred to as "prior art 2") is placed in a duct in which an evaporator can be mounted and has an air resistance equivalent to that of the evaporator.
According to the prior art 1, the air passages are defined at equal intervals in the flow-rectifying plate. The speed of air delivered under pressure from the blower unit to the heater duct varies depending on the position of a blower fan and the position, shape, and dimensions of a duct joint, but is highest at the center of the heater duct. Therefore, the flow-rectifying plate according to the prior art 1 is unable to obtain a uniform air speed distribution, and cannot deliver air at a uniform temperature and a desired rate from all the air outlets in the passenger's compartment.
Furthermore, because the flow-rectifying plate is positioned at the intermediate position in the heater duct, air streams that have passed through the flow-rectifying plate are combined with each other in the heater duct. As a result, a change is induced in the air speed distribution in the heater duct, making it highly difficult to achieve a uniform air speed distribution in the heater unit.
When air is delivered from the air blower with an evaporator installed in position in general air-conditioners, the speed distribution of air that has passed through the evaporator is often localized due to the position of the air blower. It is necessary to create such a localized air speed distribution even when the evaporator is not installed because the layout of various related components is designed taking into account the evaporator installed in position and a desired heating capability would not be achieved if the same localized air speed distribution were not obtained when the evaporator is not installed. The flow-rectifying plate according to the prior art 1, however, fails to obtain the same air speed distribution when the evaporator is not installed as the air speed distribution when the evaporator is installed.
The resistive body according to the prior art 2 which is placed in the duct has substantially the same outer profile and air resistance as the evaporator from a practical standpoint. While the resistive body can impose a desired air resistance, air that has passed through the heater duct does not have a desired air speed distribution at the inlet of the heater unit. Therefore, the heating apparatus cannot discharge air from the air outlets in the passenger's compartment at a desired temperature and a desired rate, and fail to provide a desired heating capability.